The present invention relates, in general, to electronics and, more particularly, to control circuits and methods.
In the past, TRIAC dimmers have been used to change voltage levels to loads coupled to Alternating Current (AC) power sources. In the lighting field, the TRIAC dimmers allow dimming the light intensity to a desired level. Dimming light sources results in flicker as the intensity of the light signal decreases, i.e., as it becomes dimmer. For incandescent light bulbs the issue with flicker may be solved by connecting a resistive load at the output of the dimmer. This technique is unsuitable for Light Emitting Diode (LED) light sources because the TRIAC dimmer will turn off in response to the switching current falling below the holding current of the TRIAC dimmer. U.S. Pat. No. 7,872,427 B2 issued to Carlo Scianna on Jan. 18, 2011, describes a technique for mitigating flicker in an LED by including a dummy load that turns on before the switching current falls below the holding current of the TRIAC dimmer, thereby inhibiting the TRIAC dimmer from turning off. A drawback with this approach is that the holding current varies from dimmer to dimmer. Thus, the duty cycle of the TRIAC dimmer becomes unstable and generates variability in the switching current during the AC cycle which leads to flicker in the LED. U.S. Pat. No. 8,492,992 B2 issued to Hirokazu Otake et al. on Jul. 23, 2013, describes another technique for mitigating flicker in an LED by delaying when the dummy load turns on. A drawback with this approach is that ripple is created in the LED current.
Accordingly, it would be advantageous to have a circuit and a method for mitigating the effects of a switching current falling below the holding current of a TRIAC dimmer. It is desirable for the circuit and method to be cost and time efficient to implement.
For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference characters in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or an anode of a diode, and a control electrode means an element of the device that controls current flow through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain n-channel or p-channel devices, or certain n-type or p-type doped regions, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with embodiments of the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action and the initial action. The use of the words approximately, about, or substantially means that a value of an element has a parameter that is expected to be very close to a stated value or position. However, as is well known in the art there are always minor variances that prevent the values or positions from being exactly as stated. It is well established in the art that variances of up to about ten per cent (10%) (and up to twenty per cent (20%) for semiconductor doping concentrations) are regarded as reasonable variances from the ideal goal of exactly as described.
It should be noted that a logic zero voltage level (VL) is also referred to as a logic low voltage or logic low voltage level and that the voltage level of a logic zero voltage is a function of the power supply voltage and the type of logic family. For example, in a Complementary Metal Oxide Semiconductor (CMOS) logic family a logic zero voltage may be thirty percent of the power supply voltage level. In a five volt Transistor-Transistor Logic (TTL) system a logic zero voltage level may be about 0.8 volts, whereas for a five volt CMOS system, the logic zero voltage level may be about 1.5 volts. A logic one voltage level (VH) is also referred to as a logic high voltage level, a logic high voltage, or a logic one voltage and, like the logic zero voltage level, the logic high voltage level also may be a function of the power supply and the type of logic family. For example, in a CMOS system a logic one voltage may be about seventy percent of the power supply voltage level. In a five volt TTL system a logic one voltage may be about 2.4 volts, whereas for a five volt CMOS system, the logic one voltage may be about 3.5 volts.